1. Field of the Invention
The present invention relates to devices for detecting displacement of movable bodies.
2. Description of the Related Art
To date, optical encoders have been used for detecting the positions of lenses in cameras, the rotational displacements of manual rings, and the like.
FIGS. 17A and 17B illustrate principal parts V of a rotary encoder according to a first conventional technology installed in an interchangeable lens for single-lens reflex cameras. The central line Z shown in FIG. 17A corresponds to the optical axis of the interchangeable lens.
A main scale 620 is indirectly connected to a cam (not shown) that drives the lens to move straight. The position of the lens that moves straight is detected by detecting the rotational angle of the main scale 620.
FIG. 17B is a partly enlarged view of the principal parts V shown in FIG. 17A. An index scale 630 faces the main scale 620. A light-emitting element 601 and a photointerrupter including a light-receiving element 604 are disposed so as to have the two scales interposed therebetween, and the rotational angle of the main scale 620 is detected by using the light-emitting element 601 and the photointerrupter.
According to this conventional technology, the encoder is of a transmissive type, and can have an angular U-shaped holder that supports the light-emitting element and the light-receiving element such that the two scales are clamped. Therefore, the size of encoder in a direction approximately perpendicular to the optical-axis direction is large, and it is difficult to reduce the overall size of the encoder. Moreover, the number of parts is large, resulting in poor assembling workability.
According to another conventional technology, the rotational angle is detected by using a reflective scale disposed on a cylinder.
According to a second conventional technology (Japanese Patent Laid-Open No. 5-203465), a detecting sensor is disposed outside or inside a cylindrical body.
In this second conventional technology, a reflective scale that is fixed to a columnar or cylindrical body and a reflective sensor are used for detection. Therefore, size reduction of this encoder is easier than that according to the first conventional technology.
However, sufficient resolution cannot be achieved using only the above-described structure. Moreover, these components are designed on the basis of a flat reflective scale, and loss in a light amount, displacement in a detection pitch, sensitivity in a gap characteristic, and the like caused by the deflection of light beams according to the influence of the curved reflecting surface are not discussed. Therefore, errors in an output-signal characteristic are generated by the radius of curvature, and desired performance cannot be achieved only by the second conventional technology.
Moreover, according to a third conventional technology (Japanese Patent Laid-Open No. 59-061711), a detecting sensor is disposed outside a cylinder such that the optical detection system is improved for high-precision detection.
However, according to the third conventional technology, the thickness of the detecting sensor is increased.
Recently, sensors capable of detecting positions with high resolution and high precision on the order of microns have been required for lens barrels installed in interchangeable lenses for digital single-lens reflex cameras, digital video cameras, and digital still cameras.
Moreover, smaller detecting sensors have become strongly required as the size of cameras has become smaller.